The main responsibility for the Ambulatory ECG Core is to provide analytical support for other projects which are designed to assess the impact of air pollution on vulnerability to cardiac arrhythmias. The ECG will be assessed both in terms of conventional endpoints including heart rhythm and ST-segment and QRST morphology, and for autonomic tone and cardiac vulnerability. The latter will be determined using the spectral analytical techniques of heart rate variability (HRV) and T-wave alternans, respectively. In the case of Project #1, the analyses will be directed to measuring the effects of air pollution on patients with documented myocardial infarction initially, and for subsequent groups as required. For Project 2, the Core will be a resource of expertise in cardiology. For Project 3, the analyses will provide information concerning the effect of air pollution on the free-living elderly. HRV provides a measure of autonomic balance, an important factor influencing cardiac vulnerability and a possible mediator of the deleterious influences of air pollution. T-wave alternans is a beat-by-beat variation in the area and morphology of that waveform and has been shown in numerous experimental studies to be predictive of ventricular fibrillation during diverse physiologic and pharmacologic interventions. HRV and T- wave alternans have been shown in clinical investigations to be valuable in risk stratification in diverse forms of cardiac disease. Ambulatory BCO data will be analyzed with a high capacity, state-of-the-art MARS Unity Workstation, which runs validated software for all of the standard ECG endpoints and for HRV analysis in time and frequency domains. We have recently developed and validated the software for complex demodulation of the T-wave to quantify the level of alternans and have employed the method in records of ambulatory patients with angina and silent ischemia. Thus, with the availability of this system and the involvement of individuals who are highly experienced in its use, it will -be possible to process and analyze sizeable numbers of ECG data which will be generated by Projects l & 3. Ultimately, this comprehensive hypothesis- directed approach to ECG analysis should provide important insights regarding the effects of air pollution on cardiac vulnerability and should help to elucidate the underlying mechanisms.